Rotons and roton wave packets in superfluid He-4

We give new information on the excitations in superfluid He-4. By a variational computation we obtain a very accurate description of the excitations; the energy spectrum has deviation from experiment at the level of 5%. The first microscopic study of roton wave packets is presented. R(+) and R(-) rotons cannot be described as single particle excitations due to interference effects between neighboring atoms. The roton minimum corresponds to the special wave vector where this interference disappears and the local density and momentum essentially coincide with those of a single particle excitation.